Heteroepitaxy of Ge on Si is a research topic of significant technological importance. Germanium layers integrated into Si-based complementary metal-oxide semiconductor (CMOS) technology offer attractive application possibilities.
The large lattice mismatch between Ge and Si (4%) makes it difficult to obtain high quality crystalline Ge on Si substrates. The lattice mismatch leads to a high surface roughness due to island growth and high densities of threading dislocations in the Ge epilayers. At too high temperatures Ge will mix with Si and SiGe is formed.
In “Solid phase epitaxy of deposited amorphous Ge on GaAs”, Appl. Phys. Lett. 47(8), 1985, pp. 815-817, C. J. Palmstrøm et al. describe solid phase epitaxial growth of electron beam deposited amorphous germanium on GaAs. The GaAs substrates are first cleaned to remove contaminants from the surface. Then an amorphous germanium layer is deposited onto the surface. Next, the substrates are annealed to epitaxially crystallise the amorphous germanium layer. Complete epitaxy was obtained for contamination free GaAs surfaces.
Muniz et al (J. of Physics: Condens. Matter 19(7), 076206 (2007)) studied aluminum induced growth of nanocrystalline germanium on glass substrates and on crystalline Si substrates. The influence of aluminum doping and hydrogenation of the Ge layer on the crystallization of the Ge was studied. However, the method described in this article did not lead to Ge layers with a high crystal quality. Alternatives and improved methods are constantly searched for in the industry.